This invention relates to a method for removal of residual solvent from decaffeinated coffee beans. In particular, differential pressure stripping comprises a method of flash evaporation of solvent from the beans by means of an alternating cycle of pressure variations. The time and steam requirements for desolventizing are reduced when compared to traditional methods for removal of the same solvent. This reduces the residence time of the coffee beans in conditions of high temperature and high moisture, thereby preventing negative flavor changes in the resultant coffee products.
The commercial manufacture of decaffeinated coffee requires rather lengthy process times and large amounts of steam, resulting in higher costs for decaffeinated coffees than for nondecaffeinated coffee products. Decaffeination of green coffee beans typically involves five basic well-known steps comprising steaming, pre-wetting, caffeine extracting, solvent removal, and finally drying and cooling. The desolventizing or solvent removal step traditionally required several hours to complete. Processes employing up to 20 to 40 hours for desolventizing are known. Large quantities of steam are consumed in such processes.
Process improvements in recent years have decreased desolventizing times. The improved shorter times are dependent upon the solvent employed for decaffeination. Chlorinated hydrocarbon solvents can be removed from decaffeinated coffee beans in 1 to 4 hours using conventional steam stripping. See U.S. Pat. No. 3,671,263 to Patel et al (1972). Solvents known as difficult to remove, such as ethyl acetate, require longer times of about 7 to 9 hours using the conventional steam stripping described in Patel et al. The process of the present invention reduces the time necessary for desolventizing coffee beans of ethyl acetate to about 3 to 5 hours, a decrease in time of about 50%. Steam requirements are also reduced.
Conventional steam stripping of residual solvent from coffee beans involves diffusion of solvent from the coffee beans to the steam, and is usually carried out at a constant pressure, typically below atmospheric pressure at about 2.5 to 7.5 psia. See U.S. Pat. Nos. 1,977,416 to Wilder (1934) and 3,671,263 to Patel et al (1972). It is known that decreasing pressure increases steam velocity and thereby accelerates the desolventizing step. But decreasing pressure also decreases temperature, which must be maintained at or above the boiling point of the solvent in order for steam stripping to be effective. Thus, an optimum constant pressure is normally employed dependent upon the solvent boiling point.
The British Patent Specification No. 6375 to Meyer (1906) can be interpreted as disclosing the use of alternating different pressures for removal of specific solvents. Traces of benzene or of a mixture of benzene and alcohol can be removed from decaffeinated coffee beans by subjecting the beans to a current of dry pressure steam or alternately to a current of dry pressure steam and to a vacuum, preferably while keeping the beans continually in motion. A continuous cycle of pressure change is not specifically required. However, this method was later disclosed as ineffective for full removal of ethyl acetate. Elimination of residual ethyl acetate required wet steaming of the coffee beans for 13 to 15 hours to increase their moisture content to about 30%. See U.S. Pat. No. 2,016,634 to Grethe (1935).
The desolventizing process of the present invention is distinct from the known processes in several aspects. Rather than a diffusion of solvent from a solid phase material of high solvent concentration to a gas phase of lower solvent concentration, the mechanism of the present process comprises a flash evaporation of the solvent under reduced pressure. Ethyl acetate is the preferred solvent, for which the Meyer process was disclosed as ineffective. Also, Meyer's continual agitation of the coffee beans is not used. The specific bean moisture level employed for desolventizing in the Grethe process is not required by the present invention. The use of constant pressure as in conventional steam stripping is eliminated, and both time and steam requirements are reduced. The residence time of the coffee beans in conditions of high temperature and high moisture is thereby reduced, preventing negative flavor changes in the resultant coffee products.
Accordingly, it is an object of this invention to provide an improved desolventizing process for decaffeinated coffee beans.
Another object of this invention is to reduce the amount of steam required for desolventizing decaffeinated coffee beans.
Another object of this invention is to desolventize decaffeinated coffee beans by means of an alternating cycle of different distinct pressures such that the residual solvent is flash evaporated from the beans.
It is a further object of this invention to reduce the time required for removal of residual ethyl acetate from decaffeinated coffee beans.
These and other objects will become apparent from the following description and examples. All percents are by weight unless otherwise specified.